Propylene copolymers containing from 0.1 to 10% by weight of ethylene and/or another alpha-olefin, in which the comonomer is randomly distributed in the polypropylene chain, are known as random propylene copolymers. Compared with propylene homopolymers, random propylene copolymers have a molecular structure which is modified by the presence of the comonomer, leading to a substantially lower degree of crystallinity. As a result, random propylene copolymers often have a lower melting temperature, lower sealing temperature and lower modulus of elasticity versus propylene homopolymers.
In general, lowering the total polymer crystallinity leads to a more homogenous structure in which the variation of refraction indices among the various polymer portions is minimized. The transparency of articles obtained from random copolymers is therefore increased with respect to that of the corresponding propylene homopolymers.
These characteristics make random propylene copolymers useful in the preparation of films or articles in which improved haze, impact resistance and low sealing initiation temperatures (S.I.T.) are required.
As a downside, the introduction of the comonomer into the polypropylene chain to form a random propylene copolymer leads to a significant increase in the fraction of the polymer being soluble in xylene at 25° C., with the soluble polymer portion primarily composed of lower molecular weight chains containing percentages of comonomer which are higher than the average content of comonomer in the whole polymer. The amount of soluble fraction generally increases as the average content of comonomer in the copolymer increases and, beyond defined limits, precludes the use of the copolymers in certain commercial applications, for example in the preparation of films for wrapping food, unless reduction or elimination of the soluble fraction occurs. The presence of relevant amounts of the xylene soluble fractions decreases the flowability of the polymer granules, thereby making operations such as discharging and transferring the polymer difficult and giving rise to operation problems in the polymerization plant and may leads, over the time, to the deterioration of the optical properties of the composition due to migration of these fractions to the surface (known as “blooming”).
A catalyst capable of producing low levels of soluble fractions and capable of distributing the comonomer satisfactorily in the polypropylene chain to obtain the desired effect with the lowest possible comonomer content is therefore desirable. Moreover, the catalyst should produce a copolymer with low levels of catalytic residues (Ti<15 ppm) to make additional removal stage(s) unnecessary.
It is known in the art that random propylene copolymers with improved comonomer distribution are obtainable using single-site catalysts such as those described in EP Pat. Doc. No. EP-A-318,049 and WIPO Pat. App. Pub. No WO 03/040201. These catalysts are capable of producing random propylene copolymers having a low content of xylene solubles, low melting temperatures and improved optics. However, the copolymers obtained from single site catalysts often have a very narrow molecular weight distribution, which makes them difficult to process using standard techniques and process apparati that are designed to process broader molecular weight distribution polymers such as those produced with heterogeneous Ziegler-Natta (Z-N) catalysts. Moreover, the molecular weight of the copolymers is typically low (as indicated by their relatively high melt flow index (MFI) values) and not suitable for certain applications.
However, titanium based, heterogenous Z-N catalysts generally do not randomly distribute the comonomer in and among the polymeric chains effectively, diminishing the quality of the resulting random propylene copolymers by lowering the melting such that the amount of comonomer needed makes the xylene soluble fraction undesirably high.
Some improvements in random propylene copolymer production have been described, for example, in U.S. Pat. No. 6,365,685, which relates to propylene random copolymers obtained by using a phthalate based catalyst in combination with certain 1,3-diethers as external electron donors. The random propylene polymers described therein are improved with respect to those obtained with the same phthalate-based Z-N catalysts used in combination with silanes as external electron donors. However, the properties of the random copolymers still need to be improved particularly as the xylene solubles content reported in the patent is determined by a method which comprises disadvantageously dissolving the whole sample at the boiling point of xylene, lowering the temperature of the solution to 0° C. and then let the temperature raise up to 25° C.
This method normally detects a lower amount of xylene soluble fraction with respect to the more commonly used method in which a xylene boiling solution is brought to 25° C. without lowering it to 0° C. This latter method is more challenging but is the most suitable to test the polymer's performances in terms of xylene soluble content.